This invention relates to semiconductor photoelectron emission devices.
It is possible to obtain photoelectron emission by cleaning the surface of a semiconductor and activating with cesium or cesium and oxygen. However, since the pull-out or emission probability of the electrons cannot be made sufficiently high with semiconductors whose forbidden band widths are less than about 1 eV, it has been proposed to form heterojunctions of small forbidden band width semiconductors and large forbidden band width semiconductors, and to excite the electrons from the former and emit them from the surface of the latter into the environment, such as vacuum. However, there may arise a high rate of loss by recombination of the excited electrons in the course of reaching the junction interface and the outer surface, so that this was difficult to carry out in actual practice.
Lattice matching between the different semiconductors has also been previously considered in order that the loss at the junction could be alleviated. For example, the lattice constants of germanium and zinc selenide are in good matching. However, since the two semiconductors will not make a solid solution in a wide range of concentrations, grain boundaries appear at the junction and form a large obstacle to injections of minority carriers. Further, since the zinc selenide is a direct transition type semiconductor, injected electrons are lost by recombination in the course of passing through this region. Consequently the region has to be made extremely thin, but this is quite difficult to obtain technically.